Austenitic stainless steel



United States Patent 3,171,738 AUSTENITHC STAENLESS STEEL William G. Renshaw and Remus A. Lula, Natrona fiei hts, and Harry E. Mcune EH, Eraclrenridge, Pa, assignors to Allegheny Ludium Steel Qorporaiion, Br'ackenridge, Pa, a corporation of Pennsylvania No Drawing. Filed June 29, 195%), Ser. No. 39,467 6 filaims. (Cl. 75-128) This invention relates to stainless steel and particularly to a new austenitic stainless steel. This application is a continuation-in-part of co-pending United States patent application Serial No. 697,774 filed November 21, 1957.

Austenitic stainless steels exhibit a combination of highly desirable properties that make them useful for a wide variety of industrial applications. These steels possess a balanced analysis of chromium and austenite promoting and stabilizing elements in iron and have an austenitic structure at room temperature. The austenitic structure and high chromium content both contribute to corrosion resistance and the relatively uniform austenitic structure also provides the steel with highly desirable mechanical properties, particularly workability. The most common type of austenitic stainless steel is commonly referred to as 188 which is a steel containing approximately 18% chromium and 8% nickel plus incidental ingredients. The ratio of nickel to chromium is critical in that the steel must have an austenitic structure at substantially all temperatures to which it will be exposed. If the chromium content plus other ferrite forming ingredients becomes too high in relation to the nickel present or nickel plus austenite promoting and stabilizing ingredients, grains of ferrite appear in the austenitic matrix. The presence of ferrite in the austenite matrix is decidedly detrimental to the hot workability of the metal. The presence of ferrite is caused by instability of the austenitic structure at relatively high temperatures and is generally referred to as delta ferrite. All such steels are exposed to temperatures where delta ferrite may form during fabrication.

Because of the continuing shortage of nickel, many fabricators of stainless steel are following the practice of substituting manganese for nickel as an austenite promoting addition. Manganese is not as effective as nickel in promoting a stable austenitic structure, particularly at higher temperatures where the formation of delta iron is likely to occur. Where manganese is wholly substituted for the nickel it is necessary to add nearly twice as much manganese to attain an austenitic structure. Also where manganese is used it is necessary to lower the chromium content in order to provide a reasonably stable austenite at high temperatures. Most fabricators have found it advantageous to merely substitute manganese for part of the nickel. By this means a more stable austenite is obtained, particularly at higher temperatures, thus avoiding the presence of excessive ferrite in the austenite matrix. Also, some producers of stainless steel have added nitrogen to the chromium-manganese and chromium-manganesenickel stainless steels in relatively small amounts, usually about .15 The addition of nitrogen serves to further stabilize the austenitic structure thus permitting a somewhat higher chromium content to be employed. The addition of nitrogen also contributes to the mechanical properties of the steel, particularly yield strength, and good hot workability. Examples of the prior mt manganese and nitrogen austenitic stainless steels are the 13% chrominor-14% manganese, 16% chromium-14% manganese-1% nickel, 17% chromium-12% manganese-2% nickel, 17% chromium9% manganese3% nickel and 17% chromium-6% manganese-4% nickel-% nitrogen types.

Although the manganese, manganese-nickel and manganese-nickel-nitrogen stabilized austenitic stainless steels "ice exhibit mechanical properties similar to the 188 type stainless steels, they have proven to be somewhat disappointing from the standpoint of corrosion resistance. It has not been possible to employ the higher chromium contents or other corrosion resistant imparting elements in these steels due to the relative instability of their austenitic structures. The corrosion resistance of these steels is commonly likened to Type 43 stainless steel which is a ferritic steel containing chromium alone as the major alloying ingredient. It is well known that the corrosion resistance of Type 430 steel is substantially inferior to that of the 188 stainless steels.

An analysis of austenitic stainless steel now has been found that possesses equivalent or superior corrosion resistance to the 18-8 stainless steels and equivalent or superior mechanical properties, particularly stress rupture properties, while employing only about half of the nickel commonly employed to effect such properties.

It is, therefore, an object of the present invention to provide an austenitic stainless steel with equivalent or superior corrosion resistance and mechanical properties to the l88 stainless steels while employing less nickel.

Another object of the present invention is to provide an austenitic stainless steel that contains from .0l% to .10% carbon, 7.5% to 9% manganese, .5% maximum silicon, 17.5 to 22% chromium, 5% to 7% nickel, 2% to 3% molybdenum, .25 to 50% nitrogen and the balance iron.

Other objects and advantageous features of the present invention will be obvious from the following description.

Although manganese may be substituted for nickel as the austenite promoting and stabilizing material it is not equivalent to nickel particularly in its stabilizing effect against the formation of delta iron at higher temperatures.

7 As pointed out hereinbefore it is necessary to employ some nickel in the composition to effect a reasonably stable austenite while employing desirable chromium additions. Also, it is highly desirable to make additions of nitrogen to such materials. The analysis of the present invention takes adavntage of the substitution of manganese for substantially one-half of the nickel employed in the 18-8 grades. The manganese may range from 7.5 to 9%, being consistent with effecting a savings of nickel and in providing a stable structure. We have found it necessary to employ from 5% to 7% nickel to effect a stable austenite, particularly where high temperature properties such as stress rupture are of some importance. Nitrogen effects a further stabilization of the austenite and hence imparts high workability if added within the range between 25% to 50%.

By employing the combination of austenite promoting and stabilizing materials within the ranges cited above we have found it possible to use chromium within the range from 17.5% to 22% while retaining a stable austenite structure. Austenitic stainless steels that contain less than about 17.5% chromium do not possess the desired high corrosion resistance properties. Excessive amounts of chromium result in a hard or brittle alloy.

Molybdenum, like chromium, is an addition that opposes the formation of a stable austenitic structure. However, in the present analysis it has been found possible to add 2% to 3% molybdenum in addition to the chromium without adversely affecting the austenitic structure. The preferred range where maximum corrosion resistance and mechanical properties are desired is from about 2% to 3%.

The primary advantage of the alloy of the present invention is its corrosion resistance which is comparable to the A181 Type 316 stainless steels which are 18% chromium, 8% nickel analyses that contain molybdenum. The range of alloying ingredients must be maintained within the critical ranges specified in order to maintain a stable austenitic structure which in itself contributes to the resistance of the alloy to corrosion. For example, as has been pointed out above, the nickel, manganese and nitrogen contents contribute to the stability of the austenitic structure, however, if any of these elements are present outside the specified and critical ranges, such compositions will not only exhibit less desirable mechanical properties, but will also possess less over-all corrosion resistance.

Other alloying elements employed are those commonly found in 18-8 stainless steels. Small additions of silicon are made that are consistent with good melting practices. Carbon is maintained at .10% maximum to be consistent with good corrosion resistance and high mechanical properties. Since methods of producing commercial heats containing less than 01% carbon are not available, the usual range of the alloy will be from .01% to .10%. Hence, the lower carbon contents permit this composition to be adapted to the ELC (extra low carbon) types of stainless steels that are resistant to intergranular corrosion or grain boundary carbide precipitation. The foregoing ranges are more clearly set forth in Table I, it being understood that where the balance is stated as iron, such balance includes incidental impurities such as sulfur, phosphorus, copper and the like. It is also to be understood that all analyses and ranges of composition set forth in this specification (as in Table I) are percentages by Weight of the alloying ingredients.

TABLE I C max Mo 2.0-3.0 Si 5 max Mn 7.5-9.0 Cr 17 5-220 Ni 5.0-7.0 N 25-.50

Illustrative heats showing test analyses that fall within and without the ranges of the present invention are shown in Table II.

TABLE II Heat No 0 Mn Si Cr N M0 N Samples from each heat were annealed at temperatures of from 1800 F. to 2300 F. to secure a substantially uniform austenitic structure. These samples were then tested to determine their corrosion resistance and mechanical properties. Samples of Type 316 stainless steel which is one of the most corrosion resistant of the 18-8 grades and which contains about 2.5% molybdenum were tested simultaneously with the present alloys to illustrate the comparative properties.

Corrosion tests Results of comparative corrosion tests are shown in Tables III, IV and V. Table III shows the corrosion resistance of these samples to immersion and acid solutions. The boiling nitric acid test is a well-known standard test for resistance to penetration by strongly oxidizing environments. Samples are suspended in boiling 65%, by weight, HNO for five 48-hour periods. At the conclusion of each period the weight loss' of the samples is converted into inches penetration per month. Immersion in sulfuric and oxalic acid solutions was for one 48-hour period and losses were also calculated in terms of inches penetration per month in the same manner as the nitric acid tests.

TABLE III Boiling 65% NitrieAcid, 5% Sul- 10% Oxalic Heat N0. Cr Avg. Five furic Acid 48-.H011r 140 F., 200 F., Periods, In./Mo. In./M0 In./M0.

It may be observed from Table III that samples from heats EC-47, EC-48, EC-SS, EC-89 and EC-66 all possess corrosion resistance properties similar to that exhibited by AISI Type 316 stainless steel. It should be noted that some of these samples (particularly those from Heat EC-48) exhibit greater resistance to the nitric acid exposure. It should also be noted that all the test samples that possess analyses that fall within the scope of the present invention show superior resistance to oxalic acid as compared to the Type 316 samples, and while not all of the tests of samples that possess analyses falling within the scope of the present invention exhibit corrosion resistance to sulfuric acid that is superior to the Type 316 tests, all are considered to be substantially equivalent since test results in this media are frequently erratic. Additionally, it should be noted that samples of the heats that possess analyses falling outside the scope of the present invention (Heats LEG-36, 1 112-8, FU1-8 and FT-8), with the expection of Heat FU4-8, exhibit greater corrosion losses than the samples, with analyses falling within the critical ranges. Samples from Heat EC-46 possess a chromium content slightly below the minimum chromium content of 17.5%, and although the boiling nitric acid test results show that these samples possessed equivalent corrosion resistance properties, it may be observed that specimens from this heat exhibited poor resistance to sulfuric and oxalic acids. Samples from Heat FU2-8 which had a nitrogen content below .25 Heat FU1-8 which had a nickel content below 5%and Heat FT100-8 which has a molybdenum content below 2%, all showed higher corrosion rates than either the samples that had analyses falling within the scope of the present invention or the Type 316 specimens. The samples of Heat FU4-8 possess a high chromium content and although their corrosion resistance is shown by Table III to be equivalent to or superior to the other tested specimens, the samples from this heat in the annealed condition had a hardness of about 25 Rockwell C, while the specimens with an analysis Within the scope of the present ranges were about 85 Rockwell B. The material in the softer condition is more desirable from the standpoint of being easier to work or mechanically deform.

Table IV below shows the results of intergranular corrosion testing, Specimens from each heat were sensitized for one hour at 1250 F. Such a heat treatment tends to cause chromium carbide precipitation at the grain boundaries and substantially lowers the resistance of the steel to intergranular corrision. The sensitized samples were then subject to the 65% boiling nitric acid tests as above. They were also subjected to a l00-hour test in boiling aqueous solution that contains approximately 10% by weight CuSO and 10% by Weight H 50 The latter test results in embrittlement and samples tested in this solution are bent over a radius equal to their thickness. A sample that does not show visible cracks upon bending is regarded as having passed the test While samples that exhibit some checking or minute cracking are regarded as having failed the test. V

Table V below shows the results of pit tests which were carried out in 10% ferric chloride and 20% salt spray tests (NaCl) at room temperature.

TABLE V 10% Ferric Chloride at 20% Salt Spray Room Temp.

1 Rust Spot72 Hours. No Rust Spots22 Days.

2 Pits in 6 Days..- No gits (500 Hour do No Pits (130 Hours) FU2- General Overall Pit;-

ting (5 Hrs).

FUl- No Pits (130 Hours)--.

FT1008 17. 28 do It can be seen from the test results that the alloys of the present invention possess generally superior resistance to oxidizing environments, both in the annealed and sensitized condition, substantially equivalent corrosion resistance in sulfuric acid and superior corrosion resistance in oxalic acid when compared to Type 316 stainless steel. It is also obvious from Table V that the present alloys possess substantially equivalent resistance to pitting as compared to Type 316 stainless steel.

Room temperature mechanical properties The room temperature mechanical properties of the heats EC-58 and EC-47 are shown below in Table VI as typical properties of this new stainless steel.

TABLE VI Ultimate 02% Elongation Modulus Heat Hardness, Tensile Ofiset in 2", pct. EXIOA No. Rb Strength, Yield p.s.i. Stu. p.s.i

EC58 87 109, 940 58, 640 49 27. 2 EC-47 85 107, 030 54, 780 51 30. 7

High temperature properties The high temperature tensile properties are shown in Table VII, and the stress rupture properties are shown in Tables VIII and IX.

TABLE VII High temperature tensile tests Testing Yield Tensile Percent Heat No. Tempera- Strength Strength, Elong.

ture, F. (2% Oil; p.s.i. (2)

set), p.s.i.

E047 1, 200 24, 980 63, 575 40. 5 1, 350 25, 950 48, 980 32. 150-58 l, 200 26, 230 65,495 41. 1, 350 26, 660 49. 330 45. 6 130-65 l. 200 30, 535 71, 295 42. 5 1, 350 30, 775 54. 725 42. 0 Type 316 1, 200 22, 000 55, 000 35. 0 I, 350 20, 000 40, 000 43. 0

TABLE VIII Stress rupture tests at 1200 F.

Stress, Rupture Percent Heat No. p.s.i. Time, Eloug.

Hrs (2) E647 45,000 24.5 i 10.0 42, 500 46. 0 9.0 40, 000 167. 0 12. 5 EC-dS 50, 000 12. 0 14. 0 45, 000 85. 5 18. 5 40, 000 227. 0 15. 5 E 0-66 50, 000 33. 5 10.0 5, 000 126. 5 9. 0

TABLE IX Stress for Rupture at 1200 F.

111- Identification Hours, 1000 Hours, p.s.i. p.s.i.

Average results of samples [rem each 40, 000-43, 000 as, coo-3s, 00s Average properties of Type 316 stainless steel 32, 000 25, 000

It is obvious from results shown in Tables VI, VII, VIII and IX that the alloys of the present invention possess equivalent or superior properties to the 18-8 stainless steels. It is particularly significant as may be observed from Table VII that the alloys of the present invention possess somewhat superior tensile properties at temperatures of 1200 F. and 1350 F. Also, as is illustrated in Tables VIII and IX the alloys of the present invention possess far superior stress rupture properties at 1200 F. when compared to Type 316 stainless steel.

The above samples are given to illustrate the properties and ranges of the alloys of the present invention and in no way limit the scope of the claims to those exact analyses and properties set forth.

We claim:

1. An austenitic stainless steel that contains 01% to .10% carbon, 2% to 3% molybdenum, .5% maximum silicon, 7.5 to 9% manganese, 17.5 to 22% chromium, 5% to 7% nickel, 25% to .50% nitrogen, and the balance iron, said steel being characterized by high corrosion resistance and good mechanical properties.

2. An austenitic stainless steel that consists essentially of from .01% to .10% carbon, 2% to 3% molybdenum, .5% maximum silicon, 7.5% to 9% manganese, 17.5% to 22% chromium, 5% to 7% nickel, .25% to 50% nitrogen, and the balance iron, said steel being characterized by high corrosion resistance and good mechanical properties.

3. An austenitic stainless steel that consists essentially of 072% carbon, 2.46% molybdenum, 23% silicon, 7.70% manganese, 19.01% chromium, 5.46% nickel, 28% nitrogen, and the balance iron, said steel being characterized by high corrosion resistance and good mechanical properties.

4. An austenitic stainless steel that contains .01% to .10% carbon, 2% to 3% molybdenum, .5% maximum silicon, 7.5% to 9% manganese, 17.5% to 22% chromium, 5% to 7% nickel, 25% to 50% nitrogen, and the balance iron, said steel being characterized by stress rupture properties of from 40,000 to 43,000 p.s.i. when tested for 100 hours at 1200 F.

5. An austenitic stainless steel that consists essentially of .10% maximum carbon, 2% to 3% molybdenum, .5% maximum silicon, 7.5% to 9% manganese, 17.5% to 22% chromium, 5% to 7% nickel, 25% to 50% nitrogen, and the balance iron, said steel being characterized by high corrosion resistance and good mechanical properties.

6. An austenitic stainless steel that consists essentially of .10% maximum carbon, 2% to 3% molybdenum, .5%

- gen, and the balance iron, said steel being characterized by high resistance to boiling nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,671,726 Jennings Mar. 9, 1954 8 2,819,161 Cupler Ian. 7, 1958 FOREIGN PATENTS 622,504 Canada June 20, 1961 Haefner et al., presented at the 62nd Annual Meeting of the Society, June 21 to 26, 1959 (11 pages). 

1. AN AUSTENITIC STAINLESS STEEL THAT CONTAINS .01% TO .10% CARBON, 2% TO 3% MOLYBDENUM, .5% MAXIMUM SILICON, 7.5% TO 9% MANGANESE, 17.5% TO 22% CHROMIUM, 5% TO 7% NICKEL, .25% TO .50% NITROGEN, AND THE BALANCE IRON, SAID STEEL BEING CHARACTERIZED BY HIGH CORROSION RESISTANCE AND GOOD MECHANICAL PROPERTIES. 